It has previously been common in well drilling to impart rotational drilling or hoisting forces to a drill string using a top drive unit mounted to a guide track extending upwardly within a rig derrick. These top drive units use either electric or hydraulic power. The units are generally large and are designed primarily for offshore rigs, although they have been installed in land rigs.
In prior applications, the rig derrick has had to be modified to some degree, usually through welding, to be able to accept the size of the machine, the track on which the top drive unit travels, and to be able to accept the torque imparted to the derrick from the top drive unit through the track. In some installations, the derrick has had to be extended to accommodate the top drive unit. In most cases, the rig's travelling blocks and/or rotary drilling swivel have had to be replaced or modified in order to work with the top drive unit.
Top drive implementation requires a reactive torque absorbing means. This is provided by a vertical guide track or a tensioned cable secured to the derrick. The tensioned cable puts high vertical loads on the derrick, forcing a deration of the hook load capacity. Guide tracks have generally been secured to the derrick in a manner such that significant reactive torque is transmitted to the derrick structure along its length, which has generally been designed for heavy vertical and only nominal torque loads; this has required structural modifications to compensate.
One type of reactive torque absorbing means that has been used specifically to reduce reactive torque loads on the derrick is disclosed in U.S. Pat. No. 2,998,084, issued to Johnson et al. This patent teaches a top drive and track having an interconnecting pivoting linkage apparatus. The linkage pivots about vertical axes; it is unable to transfer side loading and transmits pure rotational reactive torque into a torque track. The track is rigidly secured near its base to the rig, transferring torque loads thereinto.
The hereinabove described linkage type and other top drive and track assemblies require substantial distance between the track and the top drive to accommodate either the various interconnecting means or the large physical size of the drive mechanism. Clearance restrictions exist between the inside back of the derrick and the wellbore center. This limits the application of the apparatus of the prior art to a small number of derricks having large clearances.
Although an increasing number of land rigs are being manufactured or modified to accept conventional top drive units, to date all top drive installations described in the published prior art have been permanent, to the best of applicant's knowledge. Because of the physical and mechanical limitations of the top drive systems described in the prior art, a top drive drilling system has not been available on an economical and temporary basis for the vast majority of land drilling rigs and a substantially lesser number of offshore drilling rigs.
As stated, use of conventional top drives requires substantial modifications to the rig. Generally, modifications to a derrick will require structural re-certification. Modifications are time consuming and expensive, and are not usually undertaken for a temporary installation. With all the top drives described in the prior art, the purchase cost of a top drive for a permanent installation in most rigs is not economically justifiable when compared to conventional rotary drilling equipment used in the same drilling application on a competitive basis.
With this background in mind, it was the objective of the present invention to provide a compact, portable top drive system that could be temporarily installed in existing drilling rigs without structural modifications and without derating the derrick capacity.